kyosho

I have a '67 convertible. According to the officer that pulled me over for speeding, it looks like my speedometer is reading off by about 10mph at 50mph. Meaning I am travelling 60mph, but the speedometer says I am travelling 50mph. Is there anything I can check myself to correct this? Also, if I have to take it to a speedometer shop, any good ones in the southeastern Michigan area? Thanks in advance.

65again

Having the same problem with a V6 MR2, easiest thing I can think of is to borrow a friend's GPS, they're pretty doggone accurate, next best is a stopwatch and the interstate mile markers... Paul

1COOL60

If your speedometer seems to working well other than being off, you can change the driven gear at the transmission. Since the speedo is slow, you will need to go to a gear with fewer teeth. I would suggest you pull out the gear, count the teeth, and start with a gear one or two teeth smaller.

You could also install an adapter at the transmission that will correct the speedometer. This will screw on in place of the speedometer cable and then the cable screws onto the adapter. You should be able to find one for under one hundred dollars. I'm sure that will be less than taking it to a speedometer shop.

Donald #31176

If you know the rear end gear ratio and tire diameter of your car you can compute your speed . For a 205-75-15 tire (27.1") and 3.36 rear end gear
in 4th gear you are doing about 50.3 MPH @ 2100 RPM.
http://www.angelfire.com/fl/procrastination/rear.html


You can also follow a known accurate speedometer equipted car and see whar revs your turning at a known steady speed.

Trophy Blue

I tested mine when I saw the police radar trailer parked in front of my house. I found out that my speedometer was off by 4 mph.

JohnZ

Try SPECMO - they've been in business for decades.

http://www.specmo.com/

csalisbury

Speedometers (and tachs) are magnetized, and after 40 years, they will loose some of the energy causing them to read low. If you are handy, there are places that sell precalibrated speedo assemblies so you can simply swap it out, otherwise I would recommend sending it to a Corvette specific instrument rebuilder and have it rebuilt and restored.

kyosho

Thanks for all the replies. I have checked it against some others and I know that it is off. I think I'll take JohnZ's advice and take it to spemco. Now that he mentions it, I remember the name of that place from way back.

ffas23

This is how I test my speedometers also. These trailers I see all around the area at times telling people how fast they are going on a big lit up sign. Because of one of these trailers I was able to see that my Street Rod speedometer was off if I remember correctly by 7 or 8 miles per hour. I posted on this forum looking for some info on the plastic replacement Speedo gears. I already knew from years back that they were color coded and someone on the forum answered my thread with a link to a chart showing all that were available with GM part numbers. I think it cost me about $5 at a Chevy dealer and I got it right the first time. Flipped the dealer out also because I already had the part number. Hopefully that forum member will see this thread and give it out again. Wish I still had the link to give you. Pretty sure I kept the link but realized that the link was bookmarked on my old computer.

knight37128

I did not know there are magnets in them. I always thought they were mechanical.

piper

LIC has all the colored gears lined up in there book with the gear ratio

Chuck Gongloff

I'm no engineer, and all of the engineers on this forum will probably laugh at me, BUT

I've seen speedo's calibrated at Corvette Spec. of Md. Our Chapter has had a few tech sessions about this.

Brian has a large machine, a "gauss-degauss" machine. Using the machine, he can impart a certain degree of magnetism to the 2 spinning disks. You can also "de-gauss" the disks and remove the magnetism.

The two spinning disks act like a "slipping clutch". By imparting the correct degree of magnetism to the disks, they spin at the proper rate.

It can take a number of tries to get the magnetism correct.

This calibrates the speedo head correctly, and has nothing to do with using the correct driven gear in the trans. Chuck

JOliver

Speedometers are magnetized and can loose a bit...not common but it can happen.You'll see the contact points wear and they don't read as they should more than loosing the magnetic pull.
The speed reading is the result of a magnetized worm gear spinning and slighty pulls the speed cup with a hair spring.
Also on that gear is a worm ...it drives two other gears that turn the ODOMETER.
Speedos are calibrated in a 1:1 ratio ....the colored gears in the trans are the adjustment for rear end and tire ratios to make it read correctly for the set up you have.

Jason

Chuck Gongloff

Thanks, Jason. I knew a PRO would step up to the plate.

Lots of people think these speedos and tachs are "direct drive" and they're not. Chuck

mdz06vetter

first - what size tires are you running? are they the OEM-size, or not? second - are you still running the same rear-end and tranny as original?

both of these will impact what you are seeing on the speedo